CPR SMT NEARLY COLLIDED WITH SMA WHILE ON BASE LEG; DESCENDING FOR LNDG.

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MAKING VISUAL APCH TO LEFT DOWNWIND TO 34L (SMF) AT 3000' MSL. FOLLOWING MLG ON 3 MI FINAL 34L. STARTED TURN TOWARDS BASE LEG AND DETECTED MOTION TO THE LEFT. THE EARLY MODEL; SMA Y PASSED FROM THE NOSE TO LEFT WING TIP. NOT EVEN ENOUGH TIME TO TAKE EVASIVE ACTIONS TO AVOID. HAD A LEFT WINDSCREEN FULL OF SMA Y. WE WERE JUST LETTING DOWN THRU 2700' MSL WHEN I NOTICED SMA Y. IT WAS SO CLOSE I REMEMBER SEEING ITS 2 INCH N NUMBERS. WE ASKED TWR IF THEY HAD HIM ON RADAR; THEY SAID NO. WE WERE SLOWING THRU 180 KTS IN A LEFT SLIGHTLY NOSE HIGH POSITION TO SLOW DOWN FOR FINAL. SMA Y NOT ONLY PENETRATED AN ARSA; ALSO AN ATA. THE TWR EVENTUALLY HAD APCH CTL TRACK THIS ACFT. HE WAS FROM A LOCAL ARPT; ON A FLT TO RED BLUFF. THIS WAS TOO MUCH EXCITEMENT FOR ONE DAY. LANDED OK AND CALLED TWR FOR MORE INFO. ALL PLTS NEED TO MAKE SURE THEY KNOW THEIR AIRSPACE. AS A CFII; AIRSPACE KNOWLEDGE WILL BECOME AN EVEN MORE DISCUSSED TOPIC WITH STUDENTS. CAUSE OF PROBLEM: IDIOT; CARELESS; UNINFORMED; AND LOST PLT IN AIRSPACE HE SHOULD NOT HAVE BEEN IN. RULE LEARNED; KEEP YOUR EYES LOOKING FOR TFC EVEN IN WHAT APPEARS TO BE SAFE AND CTLED AIRSPACE.